I built a very simple temperature sensor node some time ago that basically reads the temp from a TMP36 and sends it to a base station (a Moteino) every 4 seconds and sleeps (the real application will be sleep for much longer, but I'm speeding things up just for testing).
I've been trying to run Moteino from Li-Po for while, but for long running applications didn't work very well because of the self-discharge I guess. Also tried 2xAA without regulator and was only able to run down to 1.2V per cell as mentioned before on this topic (maybe because the TMP36 needs at least 2.4V). Also tried using 3xAA with the regulator but didn't get much of it either, it's a shame that RFM69 does not support 5V, so I could connect the 3xAA directly!
The idea of running at very low voltages is great, but I think it's challenging to have all sensors and components running at 1.8V... maybe in the future new sensors will lower the working voltage to help us
I'm now testing some boards called Whisper Node, that I got from Kickstarter (
https://bitbucket.org/talk2/whisper-node-avr/overview). They are quite similar with the Moteino but there are loooots of tiny components. At the moment I'm running a similar basic temperature node with a single AA cell. The board has a step-up and when things are at sleep consumes a bit over 5uA from the battery, the whole board runs at 3.3V and MCU+temp.sensor seems to be happy.
The one thing I didn't like much is that many pins from the MCU are already used by the Whisper Node itself, like 2 voltage dividers, 2 leds and 2 buttons... it can be handy if you're using those, but won't be my case. Apparently you can cut the traces to free the pins for your own use, but just feels like I'm destroying the board - I guess everybody has different needs as Felix mentioned.
Well I'm just running the board for 3 weeks now and still can't notice a real voltage drop of the battery, I'll try to post something back when I get a bit more results... but I guess a step-up regulator might be the answer for lower voltages. I'll try to get a stand-alone step-up regulator, maybe this one:
https://www.pololu.com/product/2561 to replace the LDO on one of my Moteino to do some testings as well...